Frequency stabilization method and system for tunable light sources based on characteristic curve reconstruction

Abstract

A frequency stabilization method and system for tunable light sources based on characteristic curve reconstruction are provided, which relate the field of frequency stabilization technologies of modulation absorption spectrum. A set of frequency stabilization control method and system based on internal modulation absorption spectroscopy of light source is constructed, and a high-precision laser frequency stabilization method under large-amplitude and high-bandwidth frequency modulation based on frequency discrimination curve reconstruction is proposed to solve a problem that it is difficult for micro-probe laser interferometry measurement benchmark to balance large-amplitude and high-bandwidth frequency modulation, and high-precision frequency stabilization, resulting in that it is difficult to obtain high relative accuracy measurement under large-range measurement. Under the large-amplitude and high-bandwidth frequency modulation, a distortion model of the frequency discrimination curve and a distortion correction model are constructed, which is used for feedback adjustment of phase compensation and reconstructing the frequency discrimination curve.

Claims

1. A frequency stabilization method for tunable light sources based on characteristic curve reconstruction, comprising: step 1, outputting, by a laser (2), laser to a fiber optic coupler (4), and dividing, by the fiber optic coupler (4), the laser into two laser beams, wherein one of the two laser beams is configured to be emergent light, the other of the two laser beams is configured to be signal light to be input into an acetylene absorption chamber (5) and a photodetector (6); converting, by the photodetector (6), the signal light to an electrical signal and inputting the electrical signal into an analog-to-digital converter (7), converting, by the analog-to-digital converter (7), the electrical signal to a light intensity signal, and inputting the light intensity signal into a frequency stabilization control system (8); step 2, adjusting, by changing a driving temperature of the laser (2), an output wavelength of the laser to be approximate to a target absorption peak of acetylene gas, multiplying, by a multiplier (11), the light intensity signal with a carrier signal to obtain a multiplied signal, inputting the multiplied signal into a low-pass filter (12) to obtain a coarse frequency discrimination signal H*(), and calculating, by a peak detection unit (13), a coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*(); step 3, adjusting, by a phase compensation unit (14), a carrier phase to be compensated * of the carrier signal with a step size of 30, performing a process of multiplying, by a multiplier (11), the light intensity signal with a carrier signal to obtain a multiplied signal, inputting the multiplied signal into a low-pass filter (12) to obtain a coarse frequency discrimination signal H*(), and calculating, by a peak detection unit (13), a coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() in the step 2 for a plurality of times to obtain a plurality of coarse difference values G*, fitting the plurality of coarse difference values G* to obtain a maximum coarse difference value, to thereby obtain a coarse carrier compensation phase .sub.0 corresponding to the maximum coarse difference value; and adjusting, by using the coarse carrier compensation phase .sub.0 as an adjustment center, the carrier phase to be compensated * of the carrier signal with a step size of 5 within a range of 15 before and after the coarse carrier compensation phase .sub.0, performing the process of multiplying, by a multiplier (11), the light intensity signal with a carrier signal to obtain a multiplied signal, inputting the multiplied signal into a low-pass filter (12) to obtain a coarse frequency discrimination signal H*(), and calculating, by a peak detection unit (13), a coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() in the step 2 for a plurality of times to obtain a fine carrier compensation phase , a corresponding fine sum value D and a corresponding maximum fine difference value G.sub.max thereof; step 4, multiplying, by a frequency locking point calculation unit (15), the fine sum value D obtained in the step 3 with a scale factor k to obtain a reset frequency locking point H(.sub.0); and step 5, outputting, by a proportional integral (PI) controller (17), a controlling signal according to an error result obtained after obtaining the reset frequency locking point H(.sub.0), wherein the controlling signal is configured to adjust a driving current and the driving temperature of the laser (2), to thereby stabilize an output frequency of the laser (2) at a target frequency locking point; wherein in the step 1, a modulated output frequency of the laser (2) is expressed as .sub.m=+.sub.m cos(.sub.0t), and the light intensity signal is expressed as I[+.sub.m cos(.sub.0t)], and a Taylor formula is used to expand the light intensity signal at the output frequency of the laser (2) and combine similar terms, which is expressed as follows: $\begin{array}{c}I\left[v+v_m\cos \left({}_{0}t\right)\right]=I\left(v\right)+v_m\cos \left({}_{0}t\right)\frac{I}{v}+\frac{v_m^2}{2!}{\cos }^2\left({}_{0}t\right)\frac{{}^{2}I}{v^2}+\\ \frac{v_m^3}{3!}{\cos }^3\left({}_{0}t\right)\frac{{}^{3}I}{v^3}+\frac{v_m^4}{4!}{\cos }^4\left({}_{0}t\right)\frac{{}^{4}I}{v^4}+.Math.\\ =\left[I\left(V\right)+\frac{v_m^2}{4}\frac{{}^{2}I}{v^2}+\frac{v_m^4}{64}\frac{{}^{4}I}{v^4}+.Math.\right]+\\ \cos \left({}_{0}t\right)\left[v_m\frac{I}{v}+\frac{v_m^3}{8}\frac{{}^{3}I}{v^3}+.Math.\right]+\\ \cos \left(2{}_{0}t\right)\left[\frac{v_m^2}{4}\frac{{}^{2}I}{v^2}+\frac{v_m^2}{48}\frac{{}^{4}I}{v^4}+.Math.\right]+\\ \cos \left(3{}_{0}t\right)\left[\frac{v_m^3}{24}\frac{{}^{3}I}{v^3}+\frac{v_m^5}{384}\frac{{}^{5}I}{v^5}+.Math.\right]\end{array}$ wherein .sub.m represents a modulation frequency range, t represents a time variable, and .sub.0 represents a modulation frequency; and the coarse frequency discrimination signal H*() obtained by multiplying the light intensity signal with a carrier signal to be compensated cos(.sub.0t*) after phase adjustment is expressed as follows: $\begin{array}{c}{H}^{*}\left(v\right)=\mathrm{LPF}[I\left[v+v_m\cos \left({}_{0}t\right)\right].Math.\cos \left({}_{0}t\right)-{}^{*})]\\ \cos \left({}_c-{}^{*}\right)v_m\frac{I}{}+m\cos \left({}_m+{}_c-{}^{*}\right)I\left(\right)\\ =\cos \left({}_c-{}^{*}\right)v_m\frac{2a{}^2\left(-v_0\right)}{{\left[{}^2+{\left(-v_0\right)}^2\right]}^2}+\\ m\cos \left({}_m+{}_c-{}^{*}\right)\left(1-\frac{a{}^2}{{}^2+{\left(v-v_0\right)}^2}\right)\\ =H_o^{*}\left(\right)+H_e^{*}\left(\right)\end{array};$ wherein * represents the carrier phase to be compensated; after expanding the coarse frequency discrimination signal H*(), the coarse frequency discrimination signal comprises a coarse odd function term H.sub.o*() and a coarse even function term H.sub.e*() with a center of $v=v_0;\mathrm{and}\frac{I}{v}$ represents a first derivative of the light intensity signal I with respect to the output frequency of the laser (2), .sub.0 represents a traceable frequency corresponding to the target absorption peak of the acetylene gas, a represents an absorption rate of the target absorption peak of the acetylene gas, represents a half width of an absorption spectral line of the target absorption peak of the acetylene gas, .sub.c represents a phase delay between a frequency modulation signal of laser and the carrier signal due to optical path delay, the frequency modulation signal of laser and the carrier signal have a same frequency, m represents an adjoint light intensity modulation (AOIM) coefficient, .sub.m represents a phase difference between laser frequency modulation and light source power modulation due to adjoint light intensity, and a represents the fine carrier compensation phase.

2. The frequency stabilization method for tunable light sources based on characteristic curve reconstruction as claimed in claim 1, wherein in the step 2 and the step 3, a step of the calculating, by a peak detection unit (13), a coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() is performed for a plurality of times to iterate the coarse frequency discrimination signal H*(), to thereby achieve a carrier signal phase compensation function of the carrier signal; and the step of the calculating, by a peak detection unit (13), a coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() specifically comprises: calculating, according to an odd function characteristic of the coarse frequency discrimination signal H*() and by solving two zero frequencies .sub.1 and .sub.2 of a first derivative of the coarse odd function term H.sub.o*(), the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() by the peak detection unit (13), and formulas of the coarse peak value MAX* and the coarse valley value MIN* are expressed as follows: ${\mathrm{MAX}}^{*}=H^{*}\left(v_1\right)=\cos \left({}_c-{}^{*}\right)v_m\frac{3\sqrt{3}a}{8}+m\cos \left({}_m+{}_c-{}^{*}\right)\left(1-\frac{3a}{4}\right);$ $\mathrm{and}$ ${\mathrm{MIN}}^{*}=H^{*}\left(v_2\right)=\cos \left({}_c-{}^{*}\right)v_m\left(-\frac{3\sqrt{3}a}{8}\right)+m\cos \left({}_m+{}_c-{}^{*}\right)\left(1-\frac{3a}{4}\right);$ solving the coarse sum value D* and the coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() through the following formulas: $G^{*}={\mathrm{MAX}}^{*}-{\mathrm{MIN}}^{*}=H^{*}\left(v_1\right)-H^{*}\left(v_2\right)=\cos \left({}_c-{}^{*}\right)v_m\left(\frac{3\sqrt{3}a}{4}\right);$ $\mathrm{and}$ $D^{*}={\mathrm{MAX}}^{*}+{\mathrm{MIN}}^{*}=H^{*}\left(v_1\right)+H^{*}\left(v_2\right)=m\cos \left({}_m+{}_c-{}^{*}\right)\left(2-\frac{3a}{2}\right);$ setting a target phase step size to satisfy a target precision requirement, repeating the step of the calculating, by a peak detection unit (13), a coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() to iterate the coarse frequency discrimination signal H*(), to thereby obtain the maximum fine difference value G.sub.max and the corresponding fine carrier compensation phase thereof.

3. The frequency stabilization method for tunable light sources based on characteristic curve reconstruction as claimed in claim 2, wherein in the step 4, the traceable frequency .sub.0 is substituted into H*() through the frequency locking point calculation unit (15) to obtain a relationship formula between the reset frequency locking point H(.sub.0) and the fine sum value D, and the relationship formula is expressed as follows: $H^{}\left(v_0\right)=m\cos \left({}_m+{}_c-\right)\left(1-a\right)=D/\left(2-\frac{3a}{2}\right)\left(1-a\right)=D/k;$ wherein k represents the scale factor, and the reset frequency locking point is obtained through the fine sum value D and the scale factor k.

4. A frequency stabilization system for tunable light sources based on characteristic curve reconstruction, configured to implement the frequency stabilization method for tunable light sources based on characteristic curve reconstruction as claimed in claim 1, wherein the system comprises: a direct digital frequency synthesizer (1), the laser (2), a fiber optic isolator (3), the fiber optic coupler (4), the acetylene absorption chamber (5), the photodetector (6), the analog-to-digital converter (7), the frequency stabilization control system (8), a digital-to-analog converter (9) and a driver (10) sequentially connected in that order, wherein an output end of the driver (10) is connected to the laser (2); wherein the direct digital frequency synthesizer (1) is configured to generate a sinusoidal signal to perform current modulation on the laser (2), the laser (2) is configured to generate the laser to be input into the fiber optic isolator (3) through fiber optics, the fiber optic coupler (4) is configured to divide the laser into the emergent light and the signal light, the photodetector (6) is configured to convert the signal light passing through the acetylene absorption chamber (5) to the electrical signal, and the electrical signal is configured to pass through the analog-to-digital converter (7), the frequency stabilization control system (8) and the digital-to-analog converter (9) to obtain the controlling signal, to thereby adjust the driving current output from the driver (10) to the laser (2).

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 illustrates a schematic structural diagram of a frequency stabilization system for tunable light sources based on characteristic curve reconstruction according to an embodiment of the disclosure.

(2) FIG. 2 illustrates a schematic internal diagram of a frequency stabilization control system of the frequency stabilization system for tunable light sources based on characteristic curve reconstruction according to an embodiment of the disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

(3) Technical solutions in embodiments of the disclosure are clearly and completely described in conjunction with FIG. 1 and FIG. 2 in the embodiments of the disclosure below. Apparently, the described embodiments are merely some of the embodiments of the disclosure, not all of them. Unless specifically specified, technical means used in the embodiments are conventional means well-known to those skilled in the art.

(4) The disclosure constructs a set of frequency stabilization control method and system based on internal modulation absorption spectroscopy of a light source, and proposes a high-precision laser frequency stabilization method under large-amplitude and high-bandwidth frequency modulation based on frequency discrimination curve reconstruction to solve a problem that it is difficult for micro-probe laser interferometry measurement benchmark to balance large-amplitude and high-bandwidth frequency modulation, and high-precision frequency stabilization, resulting in that it is difficult to obtain high relative accuracy measurement under a large-range measurement. Under the large-amplitude and high-bandwidth frequency modulation, a distortion model of the frequency discrimination curve and a distortion correction model are constructed. The distortion correction model is used for feedback adjustment of phase compensation and reconstructing the frequency discrimination curve, and a corresponding relationship between the frequency locking point and a frequency stabilization benchmark is identified, so as to achieve locking and tracking of gas molecule absorption benchmarks, and accurately control the center frequency of large-range and high-bandwidth tuning.

(5) A frequency stabilization method for tunable light sources based on characteristic curve reconstruction is provided, and the frequency stabilization method includes the following steps 1-5.

(6) In step 1, laser is output to a fiber optic coupler 4 by a laser 2, and the laser is divided into two laser beams by the fiber optic coupler 4. One laser beam is used as emergent light, and the other laser beam is used as signal light to be input into an acetylene absorption chamber 5 and a photodetector 6. The signal light is converted to an electrical signal by the photodetector 6, and the electrical light is input into an analog-to-digital converter 7. The electrical signal is converted to a light intensity signal by the analog-to-digital converter 7. The light intensity signal is input into a frequency stabilization control system 8.

(7) In step 2, an output wavelength of the laser is adjusted to be approximate to a target absorption peak of acetylene gas by changing a driving temperature of the laser 2. The light intensity signal is multiplied with a carrier signal to obtain a multiplied signal by a multiplier 11.

(8) The multiplied signal is input into a low-pass filter 12 to obtain a coarse frequency discrimination signal H*(). A coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() are calculated by a peak detection unit 13.

(9) In step 3, a carrier phase to be compensated * of the carrier signal is adjusted b a phase compensation unit 14 with a step size of 30, and a process of multiplying, by a multiplier, the light intensity signal with a carrier signal to obtain a multiplied signal, inputting the multiplied signal into a low-pass filter to obtain a coarse frequency discrimination signal H*(), and calculating, by a peak detection unit, a coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() in the step 2 is performed for multiple times to obtain multiple coarse difference values G*. The multiple coarse difference values G* are fitted to obtain a maximum coarse difference value, to thereby obtain a coarse carrier compensation phase .sub.0 corresponding to the maximum coarse difference value. The carrier phase to be compensated * of the carrier signal is adjusted by using the coarse carrier compensation phase .sub.0 as an adjustment center with a step size of 5 within a range of 15 before and after the coarse carrier compensation phase .sub.0, and the process of multiplying, by a multiplier, the light intensity signal with a carrier signal to obtain a multiplied signal, inputting the multiplied signal into a low-pass filter to obtain a coarse frequency discrimination signal H*(), and calculating, by a peak detection unit, a coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() in the step 2 is performed for multiple times to obtain a fine carrier compensation phase , a corresponding fine sum value D and a corresponding maximum fine difference value G.sub.max thereof.

(10) In step 4, the fine sum value D obtained in the step 3 is multiplied with a scale factor k to obtain a reset frequency locking point H(.sub.0) by a frequency locking point calculation unit 15.

(11) In step 5, a controlling signal (U) is output by a PI controller 17 according to an error result obtained after obtaining the reset frequency locking point H(.sub.0), and the controlling signal is configured to adjust a driving current and the driving temperature of the laser 2, to thereby stabilize an output frequency of the laser 2 at a target frequency locking point.

(12) In an embodiment, in the step 1, a modulated output frequency of the laser 2 is expressed as .sub.m=+.sub.m cos(.sub.0t). The light intensity signal is expressed as I[+.sub.m cos(.sub.0t)], and a Taylor formula is used to expand the light intensity signal at the output frequency and combine similar terms as follows:

(13) $\begin{array}{c}I\left[v+v_m\cos \left({}_{0}t\right)\right]=I\left(v\right)+v_m\cos \left({}_{0}t\right)\frac{I}{v}+\frac{v_m^2}{2!}{\cos }^2\left({}_{0}t\right)\frac{{}^{2}I}{v^2}+\\ \frac{v_m^3}{3!}{\cos }^3\left({}_{0}t\right)\frac{{}^{3}I}{v^3}+\frac{v_m^4}{4!}{\cos }^4\left({}_{0}t\right)\frac{{}^{4}I}{v^4}+.Math.\\ =\left[I\left(V\right)+\frac{v_m^2}{4}\frac{{}^{2}I}{v^2}+\frac{v_m^4}{64}\frac{{}^{4}I}{v^4}+.Math.\right]+\\ \cos \left({}_{0}t\right)\left[v_m\frac{I}{v}+\frac{v_m^3}{8}\frac{{}^{3}I}{v^3}+.Math.\right]+\\ \cos \left(2{}_{0}t\right)\left[\frac{v_m^2}{4}\frac{{}^{2}I}{v^2}+\frac{v_m^2}{48}\frac{{}^{4}I}{v^4}+.Math.\right]+\\ \cos \left(3{}_{0}t\right)\left[\frac{v_m^3}{24}\frac{{}^{3}I}{v^3}+\frac{v_m^5}{384}\frac{{}^{5}I}{v^5}+.Math.\right]\end{array};$
where .sub.m represents a modulation frequency range and .sub.0 represents a modulation frequency.

(14) Therefore, the coarse frequency discrimination signal H*() obtained by multiplying the light intensity signal with the carrier signal to be compensated cos(.sub.0t*) after phase adjustment is expressed as follows:

(15) $\begin{array}{c}{H}^{*}\left(v\right)=\mathrm{LPF}[I\left[v+v_m\cos \left({}_{0}t\right)\right].Math.\cos \left({}_{0}t\right)-{}^{*})]\\ \cos \left({}_c-{}^{*}\right)v_m\frac{I}{}+m\cos \left({}_m+{}_c-{}^{*}\right)I\left(\right)\\ =\cos \left({}_c-{}^{*}\right)v_m\frac{2a{}^2\left(-v_0\right)}{{\left[{}^2+{\left(-v_0\right)}^2\right]}^2}+\\ m\cos \left({}_m+{}_c-{}^{*}\right)\left(1-\frac{a{}^2}{{}^2+{\left(v-v_0\right)}^2}\right)\\ =H_o^{*}\left(\right)+H_e^{*}\left(\right)\end{array};$
where * represents the carrier phase to be compensated. After expanding the coarse frequency discrimination signal H*(), the coarse frequency discrimination signal includes a coarse odd function term H.sub.o*() and a coarse even function term H.sub.e*() with a center of

(16) $v=v_0.Math.\frac{I}{v}$
represents a first derivative of the light intensity signal I with respect to the output frequency of the laser 2, .sub.0 represents a traceable frequency corresponding to the target absorption peak of the acetylene gas, and represents the fine carrier compensation phase.

(17) In an embodiment, in the step 2 and step 3, in order to achieve a carrier signal phase compensation function, the peak detection (i.e., a step of multiplying, by a multiplier (11), the light intensity signal with a carrier signal to obtain a multiplied signal, inputting the multiplied signal into a low-pass filter (12) to obtain a coarse frequency discrimination signal H*(), calculating, by a peak detection unit 13, a coarse peak value MAX* and a coarse valley value MIN* of the coarse frequency discrimination signal H*(), and a coarse sum value D* and a coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*()) needs to be performed for multiple times to iterate the coarse frequency discrimination signal H*(). According to an odd function characteristic of the coarse frequency discrimination signal H*(), the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() are calculated by solving two zero frequencies .sub.1 and .sub.2 of a first derivative of the coarse odd function term H.sub.o*() in the peak detection unit 13. Formulas of the coarse peak value MAX* and the coarse valley value MIN* are expressed as follows:

(18) ${\mathrm{MAX}}^{*}=H^{*}\left(v_1\right)=\cos \left({}_c-{}^{*}\right)v_m\frac{3\sqrt{3}a}{8}+m\cos \left({}_m+{}_c-{}^{*}\right)\left(1-\frac{3a}{4}\right);$$\mathrm{and}$${\mathrm{MIN}}^{*}=H^{*}\left(v_2\right)=\cos \left({}_c-{}^{*}\right)v_m\left(-\frac{3\sqrt{3}a}{8}\right)+m\cos \left({}_m+{}_c-{}^{*}\right)\left(1-\frac{3a}{4}\right);$ where .sub.c represents a phase delay between a frequency modulation signal of laser and the carrier signal due to optical path delay, the frequency modulation signal of laser and the carrier signal have a same frequency, m represents an adjoint light intensity modulation coefficient, .sub.m represents a phase difference between laser frequency modulation and light source power modulation due to adjoint light intensity, represents an absorption rate of the target absorption peak of the acetylene gas, and represents a half width (i.e., full width at half maximum, FWHM) of an absorption spectral line of the target absorption peak of the acetylene gas.

(19) The coarse sum value D* and the coarse difference value G* of the coarse peak value MAX* and the coarse valley value MIN* of the coarse frequency discrimination signal H*() are solved through the following formulas:

(20) $G^{*}={\mathrm{MAX}}^{*}-{\mathrm{MIN}}^{*}=H^{*}\left(v_1\right)-H^{*}\left(v_2\right)=\cos \left({}_c-{}^{*}\right)v_m\left(\frac{3\sqrt{3}a}{4}\right);$$\mathrm{and}$$D^{*}={\mathrm{MAX}}^{*}+{\mathrm{MIN}}^{*}=H^{*}\left(v_1\right)+H^{*}\left(v_2\right)=m\cos \left({}_m+{}_c-{}^{*}\right)\left(2-\frac{3a}{2}\right).$

(21) A target phase step size is set to satisfy a target precision requirement, the peak detection is repeated to iterate the coarse frequency discrimination signal H*(), to thereby obtain the maximum fine difference value G.sub.max and the corresponding fine carrier compensation phase thereof.

(22) In an embodiment, in the step 4, the traceable frequency .sub.0 is substituted into H*() through the frequency locking point calculation unit (15) to obtain a relationship formula between the reset frequency locking point H(.sub.0) and the fine sum value D, and the relationship formula is expressed as follows:

(23) $H^{}\left(v_0\right)=m\cos \left({}_m+{}_c-\right)\left(1-a\right)=D/\left(2-\frac{3a}{2}\right)\left(1-a\right)=D/k;$ where k represents the scale factor, and the reset frequency locking point is obtained through the fine sum value D and the scale factor k.

(24) The disclosure further provides a frequency stabilization system for tunable light sources based on characteristic curve reconstruction, which includes: a direct digital frequency synthesizer (DDS) 1, a laser 2, a fiber optic isolator 3, a fiber optic coupler 4, an acetylene absorption chamber 5, a photodetector 6, an analog-to-digital converter (ADC) 7, a frequency stabilization control system (MCU) 8, a digital-to-analog converter (DAC) 9 and a driver 10 sequentially connected in that order. An output end of the driver 10 is connected to the laser 2.

(25) The disclosure further provides the frequency stabilization control system, which includes a multiplier 11, a low-pass filter (LPF) 12, a peak detection (PAD) unit 13, a phase compensation unit 14, a frequency locking point calculation (FLPC) unit 15, a comparer (ECU) 16 and a PI controller 17. The peak detection unit 13 is configured to calculate the peak value and the valley value of the obtained frequency discrimination signal. The frequency locking point calculation unit 15 is configured to reset the frequency locking point.

(26) Specifically, the multiplier 11 is configured to calculate the multiplied value of the light intensity signal with the carrier signal. The low-pass filter 12 is configured to obtain a multiplied value after low-pass filtering (i.e., the frequency discrimination signal). The comparer 16 is configured to calculate a difference (i.e., the error result) between an actual frequency discrimination signal and a frequency discrimination signal after completing characteristic curve reconstruction. The PI controller 17 is configured to generate a controlling signal for changing the driving current and the driving temperature of the laser 2, to thereby achieve frequency stabilization.

(27) A specific work process of the disclosure is as follows.

(28) As shown in FIG. 1, the direct digital frequency synthesizer 1 generates a sinusoidal signal to perform current modulation on the laser 2, output light (i.e., the laser) of the laser 2 is transmitted to the fiber optic isolator 3 through fiber optics, and then the fiber optic coupler 4 divides the output light of the laser 2 into the emergent light and the signal light. The photodetector 6 converts the signal light passing through the acetylene absorption chamber 5 to the electrical signal. The electrical signal passes through the analog-to-digital converter 7, the frequency stabilization control system 8 and the digital-to-analog converter 9 to obtain a controlling signal, to thereby adjust the driving current output by the driver 10. Ultimately, stable molecular absorption of the frequency of the laser light source under the large-range and high-bandwidth tuning is achieved. Currently, a compensated frequency discrimination signal obtained by multiplying the modulated light intensity signal with the carrier signal after passing through the phase compensation unit 14 is expressed as follows:

(29) $\begin{array}{c}{H}^{}\left(v\right)=\mathrm{LP}F\left[I\left[+{}_m\cos \left({}_{0}t\right)\right].Math.\cos \left({}_{0}t-\right)\right]\\ \cos \left({}_c-\right){}_m\frac{I}{v}+m\cos \left({}_m+{}_c-\right)I\left(\right)\\ =\cos \left({}_c-\right){}_m\frac{2a{}^2\left(-{}_0\right)}{{\left[{}^2+{\left(-{}_0\right)}^2\right]}^2}+\\ m\cos \left({}_m+{}_c-\right)\left(1\frac{a{}^2}{{}^2+{\left(-{}_0\right)}^2}\right)\end{array};$ where represents the fine carrier compensation phase. The first term is an odd function centered on an axis =.sub.0, and the second term is an even function centered on the axis =.sub.0.

(30) Therefore, two zero frequencies .sub.1 and .sub.2 of the first term are obtained by using function derivation in the peak detection unit 13, thus obtaining a peak value MAX and a valley value MIN of the compensated frequency discrimination signal expressed as follows:

(31) $\mathrm{MAX}=H^{}\left(v_1\right)=\cos \left({}_c-\right)v_m\frac{3\sqrt{3}a}{8}+m\cos \left({}_m+{}_c-\right)\left(1-\frac{3a}{4}\right);$$\mathrm{and}$$\mathrm{MIN}=H^{}\left(v_2\right)=\cos \left({}_c-{}^{*}\right)v_m\left(-\frac{3\sqrt{3}a}{8}\right)+m\cos \left({}_m+{}_c-\right)\left(1-\frac{3a}{4}\right).$

(32) A difference value G and a sum value D of the peak value MAX and the valley value MIN are obtained, and formulas of the difference value G and the sum value D are expressed as follows:

(33) $G=\mathrm{MAX}-\mathrm{MIN}=H^{}\left(v_1\right)-H^{}\left(v_2\right)=\cos \left({}_c-\right)v_m\left(\frac{3\sqrt{3}a}{4}\right);$$\mathrm{and}$$D^{*}=\mathrm{MAX}+\mathrm{MIN}=H^{}\left(v_1\right)+H^{}\left(v_2\right)=m\cos \left({}_m+{}_c-\right)\left(2-\frac{3a}{2}\right).$

(34) After the peak value MAX, the valley value MIN, the difference value G and the sum value D of the peak value MAX and the valley value MIN are input into the phase compensation unit 14, The carrier phase to be compensated * of the carrier signal is adjusted with a step size of 30, the peak detection is performed for multiple times to obtain the G values (i.e., the difference values). The G values are compared to obtain a maximum G value (i.e., the maximum difference value), to thereby obtain an additional carrier compensation phase .sub.0 (i.e., the coarse carrier compensation phase) corresponding to the maximum G value. The additional carrier compensation phase .sub.0 is used as an adjustment center to adjust the carrier phase to be compensated * of the carrier signal with a step size of 5 within a range of 15 before and after the additional carrier compensation phase .sub.0. The peak detection is performed for multiple times to obtain a relatively accurate phase delay compensation value (i.e., the fine coarse compensation phase). When higher accuracy is required, a smaller compensation phase step size is required.

(35) A D value corresponding to the phase delay compensation value is input into the frequency locking point calculation unit 15, a traceable frequency .sub.0 is substituted into the H*() to obtain a relationship formula between the reset frequency locking point H(.sub.0) and the D value, and the relationship formula is expressed as follows:

(36) 0$H^{}\left(v_0\right)=m\cos \left({}_m+{}_c-\right)\left(1-a\right)=D/\left(2-\frac{3a}{2}\right)\left(1-a\right).$

(37) In the relationship formula, a represents an absorption rate of the target absorption peak of the acetylene gas, and it can be obtained by searching for information of the selected acetylene chamber. Therefore, reset of the frequency locking point is achieved.

(38) After analyzing test results of laser stabilization for many times in different time periods and optimizing the frequency stabilization system, a relative amount of fluctuation of the center frequency of the laser 2 is 2.510-8, that is, a relative accuracy of the center frequency is 510-8k=2, which indicates efficiency of the optimal frequency stabilization system proposed by the disclosure. A modulated signal amplitude is further changed to further adjust a modulated amplitude and test a wavelength stability. A test time after adjusting is 20 minutes (min) for each time, a modulation bandwidth is 1 megahertz (MHz) for each time. The final conclusion is that in a range of the modulation bandwidth of 1 MHz and the modulation amplitude of 150 MHz-2.61 gigahertz (GHz), the stability of the high-speed tuning frequency stabilization light source studied in the disclosure is not affected by modulation width changes, and the frequency stabilization control accuracy is on the order of 5108k=2. No impact of modulation amplitude changes on the system frequency stabilization accuracy was found under this frequency stabilization method, supporting the laser interferometer to maintain high relative accuracy in large-scale measurements.

(39) The above described embodiments are merely a description of some of the embodiments of the disclosure, and are not a limitation of a scope of the disclosure. Without departing from a design spirit of the disclosure, all variations, modifications, and replacements made by those skilled in the art to the technical solutions of the disclosure should fall within the scope of protection determined in claims of the disclosure.